Steam generation and enthalphy separation of flowing supercritical pressure steam



1968 E. L KOCHEY, JR 3,395,678

STEAM GENERATION AND ENTHALPY SEPARATION OF FLOWING SUPERCRITICAL PRESSURE STEAM Filed Dec. 28, 1966 2 Sheets-Sheet 1 INVENTOR.

EL. AOGf/EV Aug. 6, 1968 Filed Dec. 28, 1966 EN THALPV (BTU/LBJ STEAM GENERATION AND ENTHALPY SEPARATION OF FLOWING SUPERCRITICAL PRESSURE STEAM E. L. KOCHEY, JR 3,395,678

2 Sheets-Sheet 2 PRESSURE (95.4)

FIG. 2

INVENTOR.

[.L. KOtHEY United States Patent 3,395,678 STEAM GENERATION AND ENTHALPY SEPA- RATION OF FLOWING SUPERCRITICAL PRES- SURE STEAM Edward L. Kochey, In, Colebrook, Conn., assignor to Combustion Engineering, Inc., Windsor, Conn., a corporation of Delaware Filed Dec. 28, 1966, Ser. No. 605,405 5 Claims. (Cl. 122-406) ABSTRACT OF THE DISCLOSURE A supercritical pressure steam generator with recirculation through the waterwalls. A means for decreasing the pressure of the fluid below saturation pressure after passage through the waterwalls, and for separating the fluid into a water or low enthalpy portion, and a steam or high enthalpy portion. The low enthalpy portion is used for recirculation to the waterwall, while the high enthalpy portion is passed on to the superheater, after its pressure has been increased to approximately the original level. A device for separating the flowing stream of fluid at supercritical pressure into high and low enthalpy portions by throttling the flow, and thereby substantially increasing the velocity such that the static pressure of the fluid becomes less than the saturation pressure. Water and steam particles thereby formed are separated during a pressure recovery expansion by eccentrically expanding the flow with respect to the low enthalpy fluid takeoff.

Background of the invention This invention relates to enthalpy separation of steam at supercritical pressure and in particular to enthalpy separation in a recirculating typ'e supercritical pressure steam generator.

Recirculation systems have been superimposed on the waterwalls of once-through steam generators as illustrated in US. Patent 3,135,252 to W. W. Schroedter. This recirculation increases the mass flow rate of the water flowing through the waterwall tubes. This high mass flow rating improves the film conductance on the water side of the waterwall tubes which improves the overall heat transfer thereby producing lower tubing metal temperatures under heat absorbing conditions. This recirculation also decreases the enthalpy rise for each pound of fluid passing through the waterwall tubes thereby decreasing the temperature differential between various parallel waterwall tubes which is caused by unbalances in heat absorption.

The superheater following the waterwall section is a once through-flow section. A final steam temperature is established and at any particular load the heat absorption to the superheater defines the required temperature for the fluid entering the superheater. This, in turn, fixes the waterwall outlet temperature.

When a single-phase fluid is recirculated through the waterwalls, the outlet temperature remains constant, but the inlet temperature is increased due to the mixture of the hot fluid with the incoming cold fluid from the economizer. Therefore while the metal temperature of the waterwall tubes is decreased by the improvement in mass flow rate, they are increased due to the higher temperature level of the fluid flowing through these tubes.

Summary of the invention In my invention the temperature of the fluid passing through the waterwalls is decreased while recirculating fluid through the waterwalls. The fluid leaving the waterwalls is separated into a high enthalpy portion which is then passed to the superheater at the required superheater inlet conditions, and the low enthalpy portion which is returned for recirculation through the waterwalls. This is accomplished by reducing the pressure of the supercritical fluid below saturation pressure for the particular fluid condition, effecting separation into water and steam, and subsequently recompressing the steam in some manner. This may be accomplished by expanding a fluid through a turbine to low pressure and recompressing the steam with a turbine driven compressor. It may be accomplished by increasing the fluid velocity so that while the total pressure remains approximately constant, the static pressure is reduced below saturation with centrifugal separation being accomplished at this high velocity. Separation of a flowing stream of supercritical fluid may be separated into a high and low enthalpy portion by first increasing the velocity so that the static pressure is below saturation and then eccentrically expanding the fluid with respect to the low enthalpy ofltake.

The recirculation of the low enthalpy fluid results in lower fluid temperatures throughout the waterwall, and the separation of the high enthalpy fluid from the mixture allows the waterwall to operate with a lower outlet temperature. The fluid condition leaving the waterwall is further put into a relatively high specific heat zone so that the temperature differential between the fluid leaving various tubes is less even for the same enthalpy unbalance.

It is an object of my invention to operate a supercritical steam generator with the waterwalls at relatively low temperature.

It is a further object to operate a supercritical pressure steam generator With less temperature difference between various waterwall tubes.

It is a still further object to separate a flowing singlephase flow of supercritical pressure steam into a. high enthalpy stream and a low enthalpy stream.

FIGURE 1 is a schematic diagram of a supercritical pressure steam generator with enthalpy separation occurring intermediate the waterwalls and superheater utilizing a turbine to expand the fluid, a centrifugal separator to separate the water and steam, and a steam compressor to rec-empress the steam.

FIGURE 2 is a pressure-enthalpy diagram illustrating the steam conditions for the various operating conditions described.

FIGURE 3 illustrates a supercritical steam generator wherein separation is accomplished by increasing the velocity of the steam substantially and centrifically separating while the fluid is flowing at this high velocity.

FIGURE 4 illustrates a supercritical steam generator where the separation is accomplished by eccentrically expanding the fluid with respect to the low enthalpy offtake from a high velocity condition.

Water is supplied to the steam generator 2 at supercritical pressure of 3730 p.s.i. by feedwater pump 4 and passes through the economizer 6. This feedwater is then conveyed to the mixing vessel 8 and passed through the waterwall tubes 10 lining the walls of furnace 12.

Fuel is fired through burners 14 providing a heat source with the combustion products passing outwardly through flue 16 and over the heating surfaces located therein. The fluid leaving the waterwall tubes at an enthalpy of 860 B.t.u. is collected in the waterwall outlet header 18 and conveyed to the turbine 20 through which it is expanded from a supercritical pressure (3600 p.s.i.) to a pressure (3000 p.s.i.) below saturation. The effluent from the turbine is conveyed to the centrifugal separator 22 with the steam then passing up to the compressor 24. This compressor is driven by the turbine 20 and operates to recompress the steam to supercritical pressure (3500 p.s.i.) with the steam being conveyed to the superheating section 26 where it is heated to 1000 F. final steam temperature. This steam at supercritical pressure and full temperature 3 is then conveyed to and through the main turbine 28 which is connected to drive electric generator 30.

The water from the separator is passed to the mixing vessel 8 through recirculating pump 32 to effect recirculation through the water-walls. This pump may be controlled as desired or may be a free-floating pump as described in U.S. Patent 3,135,252 to W. W. Schroedter.

In the pressure enthalpy diagram of FIGURE 2 the fluid conditions throughout the system are illustrated. The water enters the economizer 6 at point 32 and is heated in the economizer to point 34. After mixing in the mixing vessel the mixed fluid temperature is as indicated at point 36, and the fluid is heated in the waterwall tubing 10 to point 38. As the fluid is expanded through the turbine 20, its condition changes to that illustrated by point 40 which is below saturation pressure and in the wet range. At separator 22 this mixture is separated into a low enthalpy fluid 42 and a high enthalpy fluid 44. This high enthalpy portion is compressed to the condition indicated by point 46 from which it passes through the superheater 26 where it is heated to the final condition 48. The pressure of the water from the separator 22 at condition 42 is increased in pressure by the circulating pump 32 where it is mixed with the economizer eflluent 34 to produce the waterwall inlet condition 36.

FIGURE 3 illustrates a similar steam generator, the dilference being in the means for effecting the separation into high and low enthalpy portions. Similar conditions prevail at points 32, 34, 36 and 38. The pressure reduction of the fluid leaving the waterwall is accomplished by increasing the velocity of the fluid at condition 38 such that its static pressure is reduced to the condition as indicated by point 50. With the fluid initially at a pressure of 3600 p.s.i. and a nominal velocity of about 50 f.p.s., the velocity is increased to 540 fps. This is accomplished by using a reduced flow area pipe section 52 which increases the velocity of the fluid. At this low static pressure water particles form at low enthalpy, with the remainder of the fluid being saturated steam. The mixture is then passed at high velocity to the centrifugal separator 22. In this separator the fluid is separated into a steam phase at condition 34 and a water phase at condition 42. The steam phase maintains its high velocity and the pressure is increased by passing through a pressure recovery pipe section 54 with the static pressure of the fluid being increased from point 44 to point 56. This steam is then passed through the superheater 36 where it is heated to the final steam conditions 48.

The water at condition 42 is increased in pressure by circulating pump 32 and is passed to the mixing vessel 8 where it is combined with the economizer eflluent 34. These fluids are mixed to produce the waterwall inlet fluid 36.

FIGURE 4 illustrates a generally similar steam generator. It is recognized that in the embodiment of FIGURE 1 the equipment for expanding and recompressing the steam could amount to a substantial investment. The embodiment of FIGURE 3 avoids the costly equipment, but the centrifugal separation of fluid flowing at an extremely high velocity would create substantial pressure drop which may be uneconomical in operation. The FIGURE 4 embodiment utilizes the general approach of the high velocity separation but avoids the concomitant high pressure drop. It is recognized that the eflicacy of separation is less than that of the FIGURE 3 embodiment since complete separation is not sustained.

The steam conditions for the economizer and Waterwall are generally as indicated previously. The static pressure of the waterwall efliuent at condition 38 is decreased by increasing the velocity in venturi section 60 to 540 fps. At this point the static pressure of the fluid is decreased to 3000 psi. and is represented by point 50. At this condition, which is below saturation, particles of saturated water form in the flowing fluid. This fluid continues at high velocity through the expansion section 62. The

low enthalpy takeoff portion 64 is located directly ahead of the high velocity section, while the high enthalpy takeoff portion 36 is eccentrically located with respect to the low enthalpy takeoff section. As the fluid flows through this expanding section, pressure recovery occurs. In :1 normal pressure recovery section after sufficient time the water particles will be re-evaporated and the single-phase fluid will be obtained. This phenomena, however, requires time since heat must be transferred from the steam to the water particles to obtain the uni-form single-phase mixture. Advantage is taken'of this time delay to effect separation while simultaneously effecting pressure recovery. The heavier and denser water particles will have a tendency to continue in a straight line while the steam will more readily change direction in the eccentric expansion. Therefore immediately after this expansion a lower enthalpy fluid will occur directly ahead of the high velocity than will occur in the eccentric portions. Divider plate 68 facilitates the maintenance of the separation of these two enthalpys so that the low enthalpy fluid is passed through outlet 64 with the high enthalpy fluid passing from outlet 66.

Since the separation is not as efficient as that previously described, the fluid at the steam condition 44 occurring in the throat of the venturi section 60 on recompression will be steam condition 70. This fluid is then heated to the final steam 48 and superheater 26. Similarly the water particles which occur at the throat of the venturi section 60 will be somewhat heated on recompression, and the enthalpy leaving outlet 64 will be as indicated by point 72. This low enthalpy fluid is then returned to mixing vessel 8 where it is mixed with the fluid leaving the economizer.

While I have illustrated and described a preferred embodiment of my invention it is to be understood that such is merely illustrative and not restrictive and that variations and modifications may be made therein without departing from the spirit and scope of the invention. I therefore do not wish to be limited to the precise details set forth but desire to avail myself of such changes as fall within the purview of my invention.

What I claim is:

1. A method of operating a supercritical pressure steam generator having a fluid recirculating system comprising: establishing a heat source; establishing a through-flow of water at supercritical pressure; heating said through-flow to an intermediate temperature level by passing the through-flow in heat exchange relation with the heat source; returning a first portion of the heated throughflow to a location upstream of the heat exchange location, and passing said returned portion again in heat exchange relation with said heat source; passing a second portion of the heated through-flow in second heat exchange with the heat source at supercritical pressure, thereby heating the through-flow to a final steam temperature; nd delivering said second portion to a point of use; characterized by: reducing the pressure of the heated through-flow at the intermediate temperature level to a pressure below saturation pressure corresponding to the enthalpy of the heated through-flow; separating the heated through-flow at the intermediate level into a water phase and a steam phase; utilizing the water phase as said first portion; utilizing the steam phase as said second portion; and increasing the pressure of said steam phase to above critical pressure after separation, but before passing the second portion in second heat exchange relation with the heat source.

2. A method as in claim 1 wherein said step of reducing pressure comprises passing the heated through-flow through a turbine, whereby energy may be obtained to compensate at least in part for the energy required to increase the pressure of steam.

3. A method as in claim 2 comprising the additional step of using the energy obtained from said turbine to compress the steam.

4-. A method as in claim 1 wherein the step of reducing pressure comprises: substantially increasing the velocity of the heated through-flow to such an extent that the static pressure of the fluid is less than saturation pressure; and wherein the step of separating the heated throughfiow into low and high enthalpy phases is accomplished by centrifically separating the heated through-flow at the increased velocity.

5. A method as in claim 1 wherein the step of reducing pressure comprises: substantially increasing the velocity of the heated through-flow to the extent that the static pressure of the fluid is less than saturation pressure; and wherein the step of separating the heated through-flow into high and low enthalpy phases is accomplished by eccentrically expanding the high velocity flow, withdrawing the low enthalpy phase from a location in line with the high velocity flow, and Withdrawing the high enthalpy phase from a location eccentric thereof.

References Cited UNITED STATES PATENTS 3,103,917 9/1963 Bauer et al 1221 3,259,111 7/1966 Koch 122-406 FOREIGN PATENTS 1,015,818 9/1957 Germany.

KENNETH W. SPRAGUE, Primary Examiner. 

